Digital light processing (DLP) is of interest for projection display systems, such as projecting images in conference rooms, home television systems, advertising displays, automobile dashboard and heads-up displays and other applications. The light engine of a DLP system typically includes the light source and other components required to generate light at several different colors. Individual color components are spatially modulated to generate individual pixels having selected color intensities. Additional optical elements focus the light onto a display.
A DLP system typically includes a spatial light modulator that modulates a light source in order to generate pixels at a projection surface with controlled intensity. A light valve is a type of spatial light modulator that modulates light across an array of elements. A light valve typically modulates optical transmission or reflection properties across an array. For example, a reflective liquid crystal light valve utilizes an array of liquid crystals elements to modulate the intensity of reflected light across the array. Another common type of light valve is a digital micro-mirror device chip (often known as a “digital light valve”) that has an array of movable micro-mirrors that can be individually tilted between two positions to vary the amount of light per-pixel that is reflected onto a display surface. Digital mirror devices can switch fast enough to allow a single spatial modulator to be used in a projection system operating in a color sequential mode. This can provide cost savings over per-color modulator designs with slow spatial modulators.
Conventionally a bright white light is used as the light source for digital projection systems. For example, the bright white light source is often implemented using an Ultra High Pressure (UHP) arc discharge lamp, which is a compact white light source with a very high luminance that was developed by Philips Electronics. A rotating color wheel is used to separate out red, green, and blue light from the white light source. Thus, when the red filter of the color wheel is aligned to the white light source, red light is focused onto the spatial modulator for the red color of pixels, and so one for the green and blue filters of the color wheel.
There are several drawbacks to conventional DLP systems. First, the image is sometimes not as bright as desired. Conventional white light sources produce a limited number of lumens of light. Additionally, conventional DLP systems waste a considerable amount of the light energy. Second, some attributes of the displayed image, such as color saturation, are deleteriously affected by the color wheel, which can introduce artifacts into the displayed image. Third, DLP systems include expensive optical elements.
Light emitting diodes (LEDs) are one alternative to white light sources. However, conventional LED light sources tend to be more expensive than UHP lamps. Additionally the brightness and the number of lumens that can be coupled to a display screen is typically about a factor of two lower for LEDs compared with UHP lamps. As a result, LEDs have many limitations as light sources in projection display systems
Semiconductor lasers have a number of potential advantages as light sources in display systems. Semiconductor lasers have high-brightness, low etendue, extended color gamut, and the capability for modulation. For example, several discrete lasers of different colors can be packaged to generate light at different colors.
However, prior art semiconductor lasers have several drawbacks as light sources for display systems. Compared with UHP white light sources, conventional semiconductor lasers are not cost-competitive and have a lower power (i.e., smaller total number of lumens of light). Additionally, semiconductor lasers typically have unacceptable speckle characteristics due to the high coherence of semiconductor lasers. In the context of a display system, a high degree of speckle results in light and dark patches across an image due to constructive and destructive interference from scatter centers.
In the prior art it was known that semiconductor lasers were not cost competitive with UHP lamps in many projection display applications. For example, for rear-projection televisions (RPTV) it was known that the light source must be able to provide 300 to 600 lumens of light for each color at a total cost of no more than about $100. See, e.g., K. Kincade, “Optoelectronics Applications: Projection Displays: Laser based projector target consumer market,” Laser Focus World, December 2005, the contents of which are hereby incorporated by reference. For a laser based system, 300 to 600 lumens corresponds to about 3 to 5 Watts for each color. However, in the prior art commercially available semiconductor lasers having the requisite luminance and satisfactory beam properties could not meet the total price point of $100 required for a RPTV system.
The cost of visible semiconductor lasers depends upon many factors. Nonlinear frequency conversion process may be used to generate red, green, and blue (RGB) colors. However, traditional approaches result in a complex system that is difficult to manufacture. Conventional visible high power semiconductor lasers require a variety of optical elements to maintain wavelength control, polarization control, and provide frequency conversion of a pump light source. For example, the Protera™ line of visible semiconductor lasers developed by Novalux, Inc. of Sunnyvale, Calif. is based upon an extended cavity surface emitting laser structure. An extended cavity laser designed to generate visible light includes a number of optical elements to stabilize the optical characteristics over a range of operating conditions during the life of the laser. Additionally, a nonlinear crystal may be included for frequency doubling a pump light source. The optical elements must be initially aligned and kept in proper alignment, which increases the cost and complexity of manufacturing. Generally speaking, the manufacturing cost of high power visible semiconductor lasers increases with each additional optical element added to the packaged optical device. Moreover, each optical element that requires a critical alignment adds a significant cost to the final laser assembly.
Additionally, the form factor of a semiconductor laser is also an important consideration in a projection display system. There have been dramatic reductions over time in the total size of projection display systems. See, for example, Derra et al. “UHP lamp systems for projection display applications,” J. Phys. D: Appl. Phys. 38 (2005) 2995-3010, the contents of which are hereby incorporated by reference. Miniaturization of the UHP lamp has reduced the reflector size of the UHP lamp to less than 50×50 mm2 or less than about 2 inches on a side. UHP lamps with reflectors having a diameter of 30 mm are also common, i.e., an area corresponding to a square area (for design purposes) of about one inch on a side. DLP chips are typically about 2 inches square in size with an active (micro-mirror) region less than one inch square (e.g., in the range of about 0.55″ to 0.75″ per side for some DLP chips). Thus, UHP lamps are rapidly approaching small form factors of about one to two cubic inches in size. For some microdisplay applications even smaller volumes (e.g., one cubic inch) are desirable. By way of comparison, the Protera™ line of high power visible extended cavity surface emitting semiconductor lasers developed by Novalux, Inc. of Sunnyvale Calif. generates 5 to 20 mW from an 11.6-cubic inch package having a length of about 4 inches (101.6 mm), a cross-sectional area of 1.79″×1.59″ (44.5×44.5 mm2) for each laser of a particular color. The Protera™ package includes room for wavelength control elements, such as etalons, polarization control elements, a surface emitting gain element, a frequency doubling crystal, and other control elements. However, in a projection display system a large number of Protera™ lasers at different wavelength would be required to have the range of wavelengths and total power required such that the total volume of the set of Protera™ laser would be immense compared to a conventional UHP lamp.
Another unresolved issue in the prior art is also how to best utilize semiconductor lasers as part of a total light engine solution. Semiconductor lasers have optical properties different from those of UHP white light lamps. Thus, a simplistic direct replacement of a UHP lamp with semiconductor lasers may not fully exploit the potential benefits of semiconductor lasers in a projection display system.
Therefore, in light of the above-described problems the apparatus, system, and method of the present invention was developed.